The market growth for plastic packages is steadily increasing. Future predictions indicate that this trend will remain and the growth in plastic packages partly is based on capturing market shares from fiber based packages. Fiber based packages can compete with plastic packages only if material and production costs can be reduced. One of the critical properties of fiber based packages is the barrier layer or layers, their functionality and the cost efficiency during production of barrier layers. Depending on the application, the barrier will be developed so it can provide the package with one or several barrier functionalities. Fiber based packages often requires a barrier layer in order to provide resistance towards penetration or diffusion of water or humidity, oil/fat/grease, aroma and/or gases.
Traditional barrier coating methods for paper and board products have been coating, impregnation, lamination or extrusion.
One of the barrier coating methods which has been under intensive research and development during recent years is dispersion barrier coating. By applying a dispersion or emulsion of polymer with blade or curtain coating technology, it is possible to offer a technology which would replace extrusion coating. The advantages with the dispersion barrier coating technology are that it offers a possibility for online coating of paper or board at the same time as it is possible to disintegrate the barrier coated paper or board which makes it easier to recycle the used fiber based substrate.
However, creating a single barrier layer by using dispersion coating might make it difficult to achieve all required properties for the package or product thereof. Typically, hot sealability and good barrier properties are difficult to obtain simultaneously for these kinds of manufactured barriers.
Another disadvantage with production of barrier by using dispersion coating is that the stability of the dispersion must be good in order to ensure good runnability. In order to achieve good stability of a dispersion it is necessary to add stabilizing components. However, by incorporating multiple components, the barrier dispersion preparation will be more difficult.
Another characteristic of the dispersion barrier is that at the formation of the barrier a substantial amount of water is applied to the substrate. This water needs to be evaporated off and high drying energy is therefore required in order to ensure a dry barrier and complete film forming of the barrier layer. Normally, the temperature of the dry coating must be significantly higher than the glass transition temperature of the polymer in order to ensure that film formation progresses. However, the use of high drying temperatures might also cause problems with blistering or adhesion between the barrier layer and the base substrate. Another problem with high drying temperature is that the tackiness of the polymer film increases due to that the temperature often will be above the glass transition temperature.
Yet another problem with traditional dispersion barrier coating is that the viscosity is relatively low (and also the solid content) which cause high level of penetration into the base substrate. This means that not only higher amount of coating is required to ensure pinhole free coating and good barrier properties, but also the fact that high drying energy is required. For uncoated board, typically 15-25 g/m2 of dry coating is required in order to get a pinhole free surface of the barrier layer.
There is thus a need for an improved method for producing paper or board with a single or multiple barrier layers in a cost efficient way.